Does Switching From Blender Render To Cycles Mess Things Up?
All matierals in cycles use nodes (even if you set up the material in the Properties panel, it will create nodes in the node editor).
Since BI materials don't use nodes by default, when you switch to cycles from BI there won't be any BI nodes in the node tree, yet nodes will be enabled. This will make the material render as transparent.
You can disable nodes (this will restore the old BI material settings, and it will not delete any nodes) by:

Pressing the node icon in the Materials panel:


Disabling the checkbox in Node editor &gt; Header:




If you want to make a single material which works for both BI and Cycles, see Is there a way to have one material for cycles and the BI